Abstract：
:Microcystinase (MlrA) catalyzes the first and most important biodegradation step of hepatotoxic microcystin-LR (MC-LR) produced and released from cyanobacterial cells, and the underlying catalytic mechanism is not completely understood yet. MlrA was postulated previously to be a metalloprotease with an active site of H260AIH263NE265, a variant of the common metal-binding motif of HEXXH. Through comparison with representative modes in HEXXH-containing metalloproteases, molecular dynamics simulation, homology modeling, and docking, the active sites of MlrA involved in the MC-LR biodegradation by Sphingomonas sp. USTB-05 were predicted. Site-directed mutants of MlrA were constructed for verification then. The results show that MlrA is likely not a metalloprotease, but a glutamate protease belonging to type II CAAX prenyl endopeptidases. Combined with the biodegradation of MC-LR by MlrA and its mutants, a complete enzymatic mechanism for MC-LR biodegradation by MlrA is proposed: Glu172 and His205 activate a water molecule facilitating a nucleophilic attack on the Adda-Arg peptide bond of MC-LR; Trp176 and Trp201 contact the carboxylate side chain of Glu172and, by raising its pKa potentially, accelerate the reaction rates; His260 and Asn264 (located in the previous postulated active center of H260AIH263NE265) function as an oxyanion hole to stabilize the transition states. This study reveals the enzymatic mechanism of MlrA for catalyzing MC-LR in both the representative modes and the experiments of site-directed mutagenesis.
journal_name
Chemospherejournal_title
Chemosphereauthors
Xu Q,Fan J,Yan H,Ahmad S,Zhao Z,Yin C,Liu X,Liu Y,Zhang Hdoi
10.1016/j.chemosphere.2019.07.012subject
Has Abstractpub_date
2019-12-01 00:00:00pages
124281eissn
0045-6535issn
1879-1298pii
S0045-6535(19)31492-4journal_volume
236pub_type
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